This invention relates to an improved method of controlling the positioning system of a robot, especially a welding robot with a vision sensor, to track a joint.
Conventional automatic arc welding systems are preprogrammed to move the welding torch along a prescribed path in close proximity to the joint. Reliance on preprogrammed torch motion often results in unsatisfactory welds in many manufacturing applications. There are variations in the dimensions of the metal parts, and heating during welding may produce thermal distortion and warping, causing the joint to move from its original position. A sensor-based tracking system is necessary to have the robot positioning system center the torch's electrode over the joint.
Visually guided tungsten inert gas (TIG) and metal inert gas (MIG) welding systems are described in several commonly assigned patents and copending applications. The center of the arc welding torch is cleared and lenses in the upper part of the torch assembly look down over the welding electrode, focusing an image of the weld puddle and joint ahead of it on the end of a fiber optic cable. The image is relayed to a solid state television-type camera that is in turn linked to a microprocessor-based intelligent vision system that analyzes the scene presented to it. Seam tracking is done by projecting a structured light pattern, such as two parallel laser bars, onto the workpieces ahead of the electrode. The location of deviations in the light bars caused by the joint indicate its position and width, providing the vision system with the information needed to control movement of the robot to automatically follow the seam.
A navigation algorithm for centering and vectoring the positioning system along a joint formed by two parts is given in allowed copending application Ser. No. 472,796, filed Mar. 7, 1983, now U.S. Pat. No. 4,542,279 A. W. Case, Jr., C. G. Ringwall, and M. L. Pollick, "Microvector Control for Edge and Joint Following", now U.S. Pat. No. 4,542,279, the disclosure of which is incorporated herein by reference. The method involves extracting and computing the next move vector from the vision scene while the current move vector is being executed. The next vector becomes the new target for the robot's positioning system and is computed and ready for execution before execution of the current vector is complete. If the structured light pattern is two parallel bars nominally perpendicular to the seam, however, the robot successfully navigates along straight joints and those with large radius bends, but is unable to follow 90.degree. corners and small radius turns (less than 0.3 inches radius, for instance).